Numerous materials that can be used as solid fuel contain so much impurities that a beneficiation is required to make them suitable for use. Examples of such solid fuels are the various kinds of brown coal, such as sub-bituminous coal, lignite and unconsolidated brown coal, the various kinds of coal, as well as peat, wood, paper, bitumen or asphalt, etc. All these materials can often only be used as a fuel after a pretreatment.
In some cases the beneficiation aims at greatly reducing the water content of the material (brown coal, peat, wood, paper). In other cases it is in particular the ash content that is reduced in the beneficiation (brown coal, coal). Often the sulphur content has to be reduced as well (bitumen, asphalt, brown coal, coal).
It will be clear that beneficiation not only improves combustion properties, but also reduces the cost of subsequent transport of the fuel. That in some cases this may mean a considerable saving becomes clear when it is considered that for example, certain kinds of brown coal contain up to 70%w of -- mainly chemically bound -- water and up to 40%w of ash-forming constituents.
In the past, it was attempted to find means of removing either the ash, the water or the sulphur compounds. A universal and attractive process by means of which two or more of these components can be removed from the fuel simultaneously is not yet available; however, the present invention aims at providing such a process.
It has been proposed to remove sulphur compounds from coal by subjecting an aqueous suspension of coal particles at elevated temperature to a heat treatment in the presence of chemicals which cause the water-insoluble sulphur compounds from the coal to be converted into soluble compounds. However, a considerable drawback of this proposal is that the subsequent separation of the coal particles and the aqueous phase by mechanical means, such as a centrifugal filter, etc., is far from complete. It yields a moist mass of coal particles containing not less than about 30%w of water. This mass must then be de-watered thermally, the result being that the dissolved sulphur compounds present in this residual water find their way into the coal again.
An aim of the present invention is to obviate this drawback and to make the very expensive thermal de-watering step superfluous.